Process for the gas-phase polymerization of olefins

ABSTRACT

A process and apparatus for producing olefin polymers are disclosed, comprising: 
     a. polymerizing one or more olefins in the gas phase, in the presence of an olefin polymerization catalyst, whereby growing polymer particles flow along a cylindrically-shaped downward path in densified form under the action of gravity so as to form a densified bed of downward-flowing polymer particles
 
b. allowing said polymer particles to flow through a restriction of the densified bed, such restriction being positioned in a restriction zone extending from the bed upward to a distance of 15% of the total height of the densified bed; and
 
c. metering an antistatic agent through a feed line connected to the densified bed at a feed point being located in a feed zone extending from the top of the restriction upward, to a distance five times the diameter of the section of the densified bed above the restriction.

FIELD OF THE INVENTION

The present invention relates to a gas-phase process for thepolymerization of olefins carried out in a reactor comprising a zonewhere the polymer particles flow downward in packed mode so as to form adensified polymer bed. In particular, the present invention is addressedto improving the operability of such reactor by appropriately feeding anantistatic agent.

BACKGROUND OF THE INVENTION

It is known that a relevant problem to be overcome in a gas-phasepolymerization process is the formation of polymer agglomerates, whichcan build up in various places, such as the polymerization reactor andthe lines for recycling the gaseous stream. When polymer agglomeratesare originated within the polymerization reactor, there can be manyadverse effects. For example, the agglomerates can disrupt the removalof polymer from the polymerization reactor by plugging the polymerdischarge valves. Further, if the agglomerates fall and cover part ofthe fluidization grid a loss of fluidization efficiency may occur. Thiscan result in the formation of larger agglomerates which can lead to theshutdown of the reactor.

SUMMARY OF THE INVENTION

It has been found that agglomerates may be also formed as a result ofthe presence of very fine polymer particles in the polymerizationmedium. Such fine particles may be present as a result of introducingfine catalyst particles or breakage of the catalyst within thepolymerization medium. Those fine particles are believed to deposit ontoand electrostatically adhere to the inner walls of the polymerizationreactor and the associated equipment for recycling the gaseous streamsuch as, for example, the heat exchanger. If the fine particles remainactive in the absence of heat removal, they will grow in size resultingin the formation of agglomerates, also caused by the partial melting ofthe polymer itself. Those agglomerates, when formed within thepolymerization reactor, tend to be in the form of sheets. Agglomeratescan also partially plug the heat exchanger designed to remove the heatof polymerization reaction.

Several solutions have been proposed to solve the problem of formationof agglomerates during a gas-phase polymerization process. Proposedsolutions include the deactivation of fine polymer particles, thecontrol of catalyst activity and the reduction of electrostatic charge.EP 359444 describes the introduction into the polymerization reactor ofsmall amounts of an activity retarder in order to keep substantiallyconstant either the polymerization rate or the content of transitionmetal in the polymer produced. The process is said to produce a polymerwithout forming agglomerates.

U.S. Pat. No. 4,739,015 describes the use of gaseous-oxygen-containingcompounds or of liquid or solid active-hydrogen-containing compounds toprevent the adhesion of the polymer to the inner wall of thepolymerization apparatus.

U.S. Pat. No. 4,803,251 describes a process for reducing the polymersheeting utilizing a group of chemical additives, which generate bothpositive and negative charges in the reactor, and which are fed to thereactor in an amount of a few parts per million (ppm) with respect tothe amount of monomer in order to prevent the formation of undesiredpositive or negative charges.

EP 560035 discloses a polymerization process in which an anti-foulingcompound is used to eliminate or reduce the build-up of polymerparticles on the walls of the reactors, or the formation of agglomeratesof polymer particles, which may cause the fouling of pipes or otherplant components. This anti-fouling compound is preferably selected fromalkydiethanolamines, which may be fed at any stage of the gas-phasepolymerization process in an amount greater than 100 ppm by weight withrespect to the produced (co)polymer. Said anti-fouling compound iscapable, when used in a standard polymerization test of ethylene andpolypropylene mixture, to selectively inhibit the polymerization onpolymer particles smaller than 850 μm, the latter being responsible forfouling problems and polymer sheeting. Other processes for reducing theelectrostatic voltage include: (1) installation of grounding devices inthe fluidized bed; (2) ionization of gas or particles by electricaldischarge to generate ions, which neutralize electrostatic charges ontothe particles; (3) the use of radioactive sources to produce radiationcapable of generating ions which neutralize electrostatic charges ontothe particles.

An innovative gas-phase process for the olefin polymerization, whichrepresents an alternative to the fluidized bed reactor technology, isdisclosed in EP 782587 and EP 1012195. The polymerization process iscarried out in a gas-phase reactor having interconnected polymerizationzones, where the growing polymer particles flow through a firstpolymerization zone (riser) under fast fluidization or transportconditions, leave said riser and enter a second polymerization zone(downcomer) through which they flow in a densified form under the actionof gravity, leave said downcomer and are reintroduced into the riser,thus establishing a circulation of polymer between the twopolymerization zones. Also the particular gas-phase technology describedin those two patent documents may suffer from the typical drawbackscorrelated with the formation of polymer agglomerates, due to thepresence of electrostatic charges within the polymerization apparatus.It has in fact been observed tendency to formation of polymeragglomerates, especially in the second polymerization zone (downcomer).In fact, along the downcomer the polymer particles flow downward in adensified form in packed mode, and this condition favors the formationof agglomerates as it is more difficult to remove the polymerizationheat due to the limited heat transfer available. The polymeragglomerates can quickly plug the polymer discharge equipment placed atthe bottom part of the downcomer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

There is therefore a need to improve the operative conditions forfeeding an antistatic compound to a gas-phase polymerization processcomprising polymer particles flowing downward along the reactor in adensified form, so as to optimize the effect of neutralizing theelectrostatic charges in this particular type of gas-phasepolymerization reactor.

In WO 2011/029735 there is described an improved arrangement of the feedpoints of an antistatic compound in such a polymerization process,whereby the antistatic agent is metered by means of several feedinglines placed at different heights of the densified polymer bed. However,there still exist a need to further improve operation of the downcomer,particularly in the production of random copolymers of propylene withethylene and/or other alpha-olefin comonomers.

It has now been found that the above object can be achieved byappropriately modifying the way of feeding an antistatic compound to thereactor.

Thus, according to a first object, the present invention provides aprocess for producing olefin polymers, which comprises the steps of:

-   (a) polymerizing one or more olefins in the gas phase, in the    presence of an olefin polymerization catalyst, whereby growing    polymer particles flow along a cylindrically shaped downward path in    densified form under the action of gravity so as to form a densified    bed of downward-flowing polymer particles, such densified bed    comprising a bed top and a bed bottom, wherein the distance from the    bed bottom to the bed top defines the height of the densified bed;-   (b) allowing said polymer particles to flow through a restriction of    the densified bed, such restriction being positioned at a distance    of abt. 15% of the total height of the densified bed from the bottom    of the bed; and-   (c) metering an antistatic agent through a feed line connected to    the densified bed at a feed point being located in a feed zone    extending from the top of the restriction, in the direction of the    bed top, to a distance five times the diameter of the section of the    densified bed above the restriction.

Preferably, the restriction zone extends from the bed bottom to adistance of 10%, more preferably 5%, of the total height of thedensified bed.

The feed point of the antistatic is positioned above the restriction, ina feed zone extending from said restriction, in the direction of the bedtop, to a distance of preferably 4 times, more preferably 3 times, evenmore preferably 2 times the diameter of the section of the densified bedabove the restriction.

A stream of a gas, also denominated as the “dosing gas”, can be fed intothe lower part of the densified bed by means of a feed line placed at ashort distance above the afore-described restriction. For “shortdistance” it is intended a distance that is generally up to 1.5 times,preferably comprised between 0.6 and 1.3 times, more preferably between0.7 and 1.0 times, the diameter of the section of the densified bedabove the restriction.

The feed line for metering the antistatic agent and the feed line formetering the dosing gas can suitably be coincident, thus implying themetering of both fluids through a single line. The process of thepresent invention advantageously applies to any gas-phase polymerizationprocesses in which the growing polymer particles flow downward into thereactor in a densified form, so that high values of density of the solidinside the reactor are reached, these values approaching the bulkdensity of the polymer.

The “poured bulk density” of a polymer is a parameter well known to theperson skilled in the art: it can be measured according to ASTMD1895/69. The density of solid inside the reactor is defined as mass ofpolymer per volume of reactor occupied by the polymer.

Specifically, throughout the present specification the term “densifiedform” of the polymer means that the ratio between the mass of polymerand the reactor volume is higher than 80% of the “poured bulk density”of the obtained polymer. Thus, for instance, in case of a polymer bulkdensity equal to 420 Kg/m³, a “densified form” of the polymer impliesthat the polymer mass/reactor volume ratio is of at least 336 kg/m³.

The operating parameters, such as temperature and pressure, are thoseusually adjusted in a gas-phase catalytic polymerization process: thetemperature is generally comprised between 60° C. and 120° C., while thepressure can range from 5 to 50 bar.

The term “antistatic agent” is used in the present description toinclude the following compounds:

-   -   antistatic substances capable of neutralizing the electrostatic        charges of the polymer particles;    -   cocatalyst deactivators that partially deactivate the aluminium        alkyl co-catalyst, provided that they do not substantially        inhibit the overall polymerization activity.

Consequently, an “antistatic agent” according to the invention is anysubstance that is capable to prevent, eliminate or substantially reducethe formation of build-up of polymer on any equipment of thepolymerization plant, including sheeting of reactor walls, or depositsof polymer agglomerates onto any line of the polymerization plant,including the gas recycle line.

According to present invention an antistatic agent is metered into thepolymerization process according to a specific arrangement, so as tomaximise the anti-static effect of neutralizing the electrostaticcharges on the polymer particles flowing downward in a densified formalong the polymerization reactor.

According to the present invention, the antistatic agent can be added tothe polymerization process neat or diluted in a hydrocarbon solvent,which is useful to improve its dispersion. Suitable hydrocarbon solventsare isopentane, isohexane, n-hexane, cyclohexane, heptane. When asolvent is employed, the amount of antistatic agent in the solution(antistatic+solvent) may range from 2% to 60% by wt, preferably from 4%to 40% by weight.

The antistatic agent is generally added to the polymerization process ina total amount ranging from 5 to 250 ppm weight, based on the weight ofpolyolefin being produced. Use of lower amounts will be less effectivein preventing the polymer buildup, while use of larger amounts willadversely affect the operation of the reactor, more specifically thecatalyst activity. Preferred amounts of said antistatic agent are withinthe range from 10 to 100 ppm weight, based on the weight of polyolefinbeing produced.

In particular, the process of the present invention can beadvantageously applied to the gas-phase polymerization processesdescribed in EP 782587 and EP 1012195, where the polymerization of oneor more olefins is carried out in two interconnected polymerizationzones. In fact, the polymerization conditions inside the secondpolymerization zone are such that the polymer particles flow downward ina “densified form” under the action of gravity.

Therefore, according to a preferred embodiment of present invention, oneor more alpha-olefins are polymerized in a gas-phase reactor having twointerconnected polymerization zones, the first polymerization zone,denominated the riser, comprising polymer particles flowing upward underfast fluidization or transport conditions, the second polymerizationzone, denominated the downcomer, comprising polymer particles flowingdownward in densified form under the action of gravity so as to form adensified polymer bed.

Fast fluidization conditions inside the riser are established by feedinga gas mixture comprising one or more alpha-olefins at a velocity higherthan the transport velocity of the polymer particles. The velocity ofsaid gas mixture is generally comprised between 0.5 and 15 m/s,preferably between 0.8 and 5 m/s. The terms “transport velocity” and“fast fluidization conditions” are well known in the art; for adefinition thereof, see, for example, “D. Geldart, Gas FluidisationTechnology, page 155 et seq., J. Wiley & Sons Ltd., 1986”.

In the downcomer the polymer particles flow under the action of gravityin a densified form, so that the density of the solid inside thispolymerization zone approaches the bulk density of the polymer.

According to the present invention it has been observed that feeding anantistatic agent, besides being effective in reducing the formation ofpolymer sheeting and polymer agglomerates along the entire downcomer, iscrucial in keeping the values of the “skin” temperature under control,intended as the temperature measured on the external surface of thereactor by means of suitable apparatuses that are commerciallyavailable. High values of such “skin” or wall temperature indicate astagnant area where the growing polymer can, and generally will, growinto lumps thus jeopardizing the operation of the reactor.

Other additional feed lines of the antistatic agent may be arrangedaccording to the process of the invention. Particularly, in apolymerization reactor having two interconnected polymerization zonessaid additional feeds of antistatic agent may suitably be arranged alongthe downcomer, on the line for feeding the catalyst system to the riserand/or along the line that continuously recycles the gas monomers to thepolymerization reactor.

The total amount of antistatic agent added to the polymerization reactorsuitably ranges from 20 to 500 ppm weight, based on the molecular weightof the polyolefin being produced. Preferred amounts of said antistaticagent are within the range from 50 to 250 ppm weight, again based on themolecular weight of the polyolefin being produced.

The process of the present invention will now be described in detailwith reference to the enclosed FIGURE, which has to be consideredillustrative and not limitative of the scope of the invention.

FIG. 1 is a diagrammatic representation of the process of the inventionwhen applied to a gas-phase polymerization reactor having twointerconnected polymerization zones, as described in EP-B-782587 andEP-B-1012195.

The polymerization reactor shown in FIG. 1 comprises a firstpolymerization zone 1 (riser), wherein the polymer particles flow upwardunder fast fluidization conditions along the direction of the arrow Aand a second polymerization zone 2 (downcomer), wherein the polymerparticles flow downward under the action of gravity along the directionof the arrow B.

The upper portion of the riser 1 is connected to a solid/gas separator 3by the interconnection section 4. The separator 3 removes most of theunreacted monomers from the polymer particles and the polymer withdrawnfrom the bottom of separator 3 enters the top portion of the downcomer2. The separated unreacted monomers, optionally together withpolymerization diluents, such as propane, flow up to the top ofseparator 3 and are successively recycled to the bottom of the riser 1via the recycle line 5.

A mixture comprising one or more olefin monomers, hydrogen as themolecular weight regulator, propane as the polymerization diluent, isfed to the polymerization reactor via one or more lines M, which aresuitably placed along the gas recycle line 5, according to the knowledgeof the person skilled in art.

The catalyst components, preferably after a prepolymerization step, arecontinuously introduced into the riser 1 via line 6. The producedpolymer can be discharged from the reactor via a line 7, which isadvantageously placed on the lower portion of the downcomer 2: in fact,due to the packed flow of densified polymer, the quantity of gasentrained with the discharged polymer is minimized. By inserting acontrol valve (not shown in FIG. 1) on the polymer discharge line 7, itbecomes possible to continuously control the flow rate of polymerproduced by the polymerization reactor. Additional polymer dischargelines with respect to line 7 can conveniently be placed in the bottompart of the downcomer.

The polymerization reactor of present invention further comprises atransport section 8 connecting the bottom of downcomer 2 with the lowerregion of the riser 1. The bottom of the downcomer 2 converges into arestriction 9. Said restriction 9 is suitably tronco-conically shapedand its walls form a vertical angle in a range of about 5 to 15°,preferably of around 10°. A control valve 10 with an adjustable openingis conveniently placed within or just below said restriction 9. When thecontrol valve 10 is placed below said restriction, the distance betweenthe two is suitably minimized. Also the distance between the controlvalve 10 and the upper part of the transport section 8 is suitablyminimized. The flow rate Fp of polymer continuously circulated betweenthe downcomer 2 and the riser 1 is adjusted by the level of opening ofsaid control valve 10. The control valve 10 may be a mechanical valve,such as a simple or double butterfly valve, a ball valve, etc.

A stream of a gas, also denominated as the “dosing gas”, is fed into thelower part of the downcomer 2 by means of a feed line 11 placed at ashort distance above said restriction 9. Said line 11 can beconveniently split into multiple lines that can suitably be arrangedaround a section of the reactor, preferably in an even number (e.g. two,four, six, eight). The dosing gas to be introduced through line 11 isconveniently taken from the recycle line 5. In synthesis, the flow Fp ofpolymer particles circulated between downcomer 2 and riser 1 isconveniently adjusted by varying the opening of said control valve 10 atthe bottom of the downcomer and/or by varying the flow rate of saiddosing gas entering the downcomer via line 11. The flow rate of dosinggas is adjusted by means of a control valve 18, which is suitablyarranged on line 11.

According to the present invention, an antistatic agent A can be meteredinto the reactor via feed line 11. A flow rate A 1 of such antistaticagent is metered via line 19 by valve 20 and then dispersed into asuitable flow rate of liquid monomer L to obtain a more homogeneousdistribution. Such dispersion is then pre-mixed with the dosing gas andthence fed into the downcomer.

As described in International patent application WO 2011/029735, theantistatic agent can additionally be metered to one or more positionsalong the height of the downcomer via suitable nozzles. In such a case,the antistatic agent flow rate A2 in line 22 is metered by one or moresuitable valves 23 and then pre-dispersed either in the liquid monomer Las described above, or alternatively in a fraction of recycle gas takenfrom recycle line 5 via line 24. Furthermore, as also described in WO2011/029735, additional flow rates of antistatic agent may be fed intothe reactor at the bottom of the riser (flow rate A3, line 25 metered byvalve 26) or into the main gas recycle line 5 (flow rate A4, line 27metered by valve 28).

The transport section 8 is designed as a bend descending from the bottomof downcomer 2 up to the lower region of the riser 1. A carrier gas isintroduced via line 12 at the inlet of said transport section 8. Theflow rate of the carrier gas is adjusted by means of a control valve 13,which is suitably arranged on line 12.

Also the carrier gas is conveniently taken from the gas recycle line 5.Specifically, the gas recycle stream of line 5 is first subjected tocompression by means of a compressor 14 and only a minor percentage ofsaid recycle stream passes through line 12, thus entering the transportsection 8 and diluting the solid phase of polymer flowing through thetransport section 8. Most of the recycle stream, downstream thecompressor 14, is subjected to cooling in a heat exchanger 15 andsuccessively is introduced via line 16 at the bottom of the riser 1 at ahigh velocity, such to ensure fast fluidization conditions in thepolymer bed flowing along the riser 1.

The carrier gas merges with the densified polymer coming from downcomer2 at the inlet portion of transport section 8, after exiting the slitsof the gas distribution grid 17. In the embodiment shown in FIG. 1 thetop end of the distribution grid 17 is coincident with the inlet of thetransport section 8 and said distribution grid 17 extends along thebending of said transport section 8 for an angle α=60°. The gasdistribution grid 17 is formed by a plurality of trays fixed to thetransport section 8 in a way to form slits in the overlapping area ofadjacent trays. A detailed description of the gas distribution grid 17can be found in International patent application WO 2012/031986,

An additional flow rate of antistatic agent can suitably be meteredthrough line 12.

Depending on the olefin (co)polymer to be produced, the polymerizationreactor can be operated by properly adjusting the polymerizationconditions and the monomers concentration in the riser and in thedowncomer, so as to produce a wide variety of bimodal homopolymers andrandom copolymers. To this purpose, the gas mixture entraining in thepolymer particles and coming from the riser can be partially or totallyprevented from entering the downcomer, so as to polymerize two differentmonomers compositions in the riser and the downcomer. This effect may beachieved by feeding a gaseous and/or a liquid barrier stream through aline placed in the upper portion of the downcomer: said barrier streamshould have a suitable composition, different from the gas compositionpresent inside the riser. The flow rate of said barrier stream can beadjusted, so that an upward flow of gas counter-current to the flow ofthe polymer particles is generated, particularly at the top of thedowncomer, thus acting as a barrier to the gas mixture coming from theriser. For further details regarding this barrier effect at the top ofthe downcomer, reference is made to the disclosure of EP-B-1012195.

In general, all the antistatic agents conventionally known in the art,which are able to prevent, eliminate or substantially reduce theformation of build-up of polymer on any part of the polymerizationplant, may be used in the present invention. An overview of antistaticagents suitable for polymerization processes is also given in EP 107127.

The antistatic agent can be selected from one or more of the followingclasses:

-   (1) alkyldiethanolammines of formula R—N(CH₂CH₂OH)₂ wherein R is an    alkyl radical comprised between 10 and 20 carbon atoms, preferably    between 12 and 18 carbon atoms;-   (2) Polyepoxidate oils, such as epoxidate linseed oil and epoxidate    soya oil;-   (3) Polyalcohols having from 4 to 8 carbon atoms;-   (4) Hydroxyesters with at least two free hydroxyl groups, obtained    from carboxylic acids with from 8 to 22 carbon atoms and from    polyalcohols;-   (5) Amides of formula R—CONR′R″, wherein R, R′, and R″ may be the    same or different and is a saturated or unsaturated hydrocarbon    radical having 1 to 22 carbon atoms;-   (6) Fatty acid soaps represented by the general formula R—COOM,    wherein R is a saturated or unsaturated hydrocarbon radical having    12 to 22 carbon atoms, and M is an alkali or alkaline earth metal;-   (7) Salts of sulfuric acid esters of higher alcohols represented by    the general formula ROS0₃M, wherein R is a saturated or unsaturated    hydrocarbon radical having 12 to 22 carbon atoms, and M is an alkali    or alkaline earth metal;-   (8) Salts of sulfuric acid esters of higher secondary alcohols    represented by the general formula

wherein R and R′ may be the same or different and are selected fromsaturated or unsaturated hydrocarbon radical having 12 to 22 carbonatoms, M is an alkali or alkaline earth metal;

-   (9) Compounds represented by the general formula

ROCH₂CH₂_(n)OSO₃M.

-   -   wherein R, M and n are the same as above defined;

-   (10) Salts of (higher alkyl) sulfonic acids represented by the    general formula RS0₃M wherein R, M and n are the same as above    defined;

-   (11) Salts of alkylarylsulfonic acids;

-   (12) Alkali or alkaline earth metal salts of dialkylsulfosuccinic    acids;

-   (13) Alkali or alkaline earth metal salts of partial esters of    higher alcohols with phosphoric acid;

-   (14) Salts of primary amines represented by the general formula

[R—NH₃]⁺A⁻

wherein R is a saturated or unsaturated hydrocarbon radical; A ischlorine, bromine;

-   (15) Compounds of the alkylaminesulfonic acid type represented by    the general formula

-   (16) Compounds represented by the general formula

-   -   wherein R is a saturated or unsaturated hydrocarbon radical        having 4 to 22 carbon    -   atoms; n and m, which may be the same or different, are numbers        of from 1 to 10;

Preferred antistatic agents used in the process of the present inventionare the compounds belonging to the above classes (1), (2), (3), (4) and(5).

Among the compounds of class (1) particularly preferred antistaticcompounds are alkydiethanolamines, wherein the alkyl group has from 10to 18 carbon atoms. A preferred compound is a commercial product soldunder the trademark ATMER163® (mixture of alkyldiethanolammines offormula R—N(CH₂CH₂OH)₂ where R is an alkyl radical C₁₂-C₁₈).

Among the compounds of class (2) particularly preferred antistaticcompound is Edenol D81®. Among the compounds of class (4) particularlypreferred is glycerol monostearate (GMS).

The polymerization process of the invention allows the preparation of alarge number of polyolefins. Examples of polyolefins that can beobtained are:

-   -   high-density polyethylene (HDPE having relative densities higher        than 0.940) including ethylene homopolymers and ethylene        copolymers with α-olefins having 3 to 12 carbon atoms;    -   linear polyethylene of low density (LLDPE having relative        densities lower than 0.940) and of very low density and ultra        low density (VLDPE and ULDPE having relative densities lower        than 0.920 down to 0.880) consisting of ethylene copolymers with        one or more α-olefins having 3 to 12 carbon atoms;    -   elastomeric terpolymers of ethylene and propylene with minor        proportions of diene or elastomeric copolymers of ethylene and        propylene with a content of units derived from ethylene of        between about 30 and 70% by weight;    -   isotactic polypropylene and crystalline copolymers of propylene        and ethylene and/or other α-olefins having a content of units        derived from propylene of more than 85% by weight;    -   isotactic copolymers of propylene and α-olefins, such as        1-butene, with an α-olefin content of up to 30% by weight;    -   impact-resistant propylene polymers obtained by sequential        polymerisation of propylene and mixtures of propylene with        ethylene containing up to 30% by weight of ethylene;    -   atactic polypropylene and amorphous copolymers of propylene and        ethylene and/or other α-olefins containing more than 70% by        weight of units derived from propylene.

The polymerization process of the present invention can be carried outupstream or downstream other conventional polymerization technologies(either in a liquid-phase or a gas-phase), giving rise to a sequentialmultistage polymerization process. For instance, a fluidised bed reactorcan be used to prepare a first polymer component, which is successivelyfed to the gas-phase reactor of FIG. 1 to prepare a second and a thirdpolymer component. Accordingly, an ethylene polymer endowed with atri-modal molecular weight distribution can be obtained, as well as apolypropylene blend comprising three components having a differentcontent in ethylene.

The gas-phase polymerization process herewith described is notrestricted to the use of any particular family of polymerizationcatalysts. The invention is useful in any exothermic polymerizationreaction employing any catalyst, whether it is supported or unsupported,and regardless of whether it is in pre-polymerized form.

The polymerization reaction can be carried out in the presence of highlyactive catalytic systems, such as Ziegler-Natta catalysts, single sitecatalysts, chromium-based catalysts, vanadium-based catalysts.

A Ziegler-Natta catalyst system comprises the catalysts obtained by thereaction of a transition metal compound of groups 4 to 10 of thePeriodic Table of Elements (new notation) with an organometalliccompound of group 1, 2, or 13 of the Periodic Table of element.

In particular, the transition metal compound can be selected amongcompounds of Ti, V, Zr, Cr, and Hf. Preferred compounds are those offormula Ti(OR)_(n)X_(y-n) in which n is comprised between 0 and y; y isthe valence of titanium; X is halogen and R is a hydrocarbon grouphaving 1-10 carbon atoms or a COR group. Among them, particularlypreferred are titanium compounds having at least one Ti-halogen bondsuch as titanium tetrahalides or halogenalcoholates. Preferred specifictitanium compounds are TiCl₃, TiCl₄, Ti(OBu)₄, Ti(OBu)Cl₃, Ti(OBu)₂Cl₂,Ti(OBu)₃Cl.

Preferred organometallic compounds are the organo-Al compounds and inparticular Al-alkyl compounds. The alkyl-Al compound is preferablychosen among the trialkyl aluminum compounds such as for exampletriethylaluminum, triisobutylaluminum, tri-n-butylaluminum,tri-n-hexylaluminum, tri-n-octylaluminum. It is also possible to usealkylaluminum halides, alkylaluminum hydrides or alkylaluminumsesquichlorides such as AlEt₂Cl and Al₂Et₃Cl₃ optionally in mixture withsaid trialkyl aluminum compounds.

Particularly suitable high yield ZN catalysts are those wherein thetitanium compound is supported on magnesium halide in active form whichis preferably MgCl₂ in active form. Particularly for the preparationcrystalline polymers of CH₂CHR olefins, where R is a Cl C10 hydrocarbongroup, internal electron donor compounds can be supported on the MgCl₂.Typically, they can be selected among esters, ethers, amines, andketones. In particular, the use of compounds belonging to 1,3-diethers,cyclic ethers, phthalates, benzoates, acetates and succinates ispreferred.

When it is desired to obtain a highly isotactic crystallinepolypropylene, it is advisable to use, besides the electron-donorpresent in the solid catalytic component, an external electron-donor(ED) added to the aluminium alkyl co-catalyst component or to thepolymerization reactor. These external electron donors can be selectedamong alcohols, glycols, esters, ketones, amines, amides, nitriles,alkoxysilanes and ethers. The electron donor compounds (ED) can be usedalone or in mixture with each other. Preferably the ED compound isselected among aliphatic ethers, esters and alkoxysilanes. Preferredethers are the C2-C20 aliphatic ethers and in particular the cyclicethers preferably having 3-5 carbon atoms, such as tetrahydrofurane(THF), dioxane.

Preferred esters are the alkyl esters of C1-C20 aliphatic carboxylicacids and in particular C1-C8 alkyl esters of aliphatic mono carboxylicacids such as ethylacetate, methyl formiate, ethylformiate,methylacetate, propylacetate, i-propylacetate, n-butylacetate,i-butylacetate. The preferred alkoxysilanes are of formula R_(a) ¹R_(b)²Si(OR³)_(c), where a and b are integer from 0 to 2, c is an integerfrom 1 to 3 and the sum (a+b+c) is 4; R¹, R², and R³, are alkyl,cycloalkyl or aryl radicals with 1-18 carbon atoms. Particularlypreferred are the silicon compounds in which a is 1, b is 1, c is 2, atleast one of R¹ and R² is selected from branched alkyl, cycloalkyl oraryl groups with 3-10 carbon atoms and R³ is a C₁-C₁₀ alkyl group, inparticular methyl.

Other useful catalysts are the vanadium-based catalysts, which comprisethe reaction product of a vanadium compound with an aluminum compound,optionally in the presence of a halogenated organic compound. Optionallythe vanadium compound can be supported on an inorganic carrier, such assilica, alumina, magnesium chloride. Suitable vanadium compounds areVCl₄, VCl₃, VOCl₃, vanadium acetyl acetonate.

Other useful catalysts are those based on chromium compounds, such aschromium oxide on silica, also known as Phillips catalysts.

Other useful catalysts are single site catalysts, for instancemetallocene-based catalyst systems which comprise:

at least a transition metal compound containing at least one π bond;at least an alumoxane or a compound able to form an alkylmetallocenecation; and optionally an organo-aluminum compound.

A preferred class of metal compounds containing at least one π bond aremetallocene compounds belonging to the following formula (I):

Cp(L)_(q)AMX_(p)  (I)

wherein M is a transition metal belonging to group 4, 5 or to thelanthanide or actinide groups of the Periodic Table of the Elements;preferably M is zirconium, titanium or hafnium;the substituents X, equal to or different from each other, aremonoanionic sigma ligands selected from the group consisting ofhydrogen, halogen, R⁶, OR⁶, OCOR⁶, SR⁶, NR⁶ ₂ and PR⁶ ₂, wherein R⁶ is ahydrocarbon radical containing from 1 to 40 carbon atoms; preferably,the substituents X are selected from the group consisting of —Cl, —Br,-Me, -Et, -n-Bu, -sec-Bu, -Ph, -Bz, —CH₂SiMe₃, -OEt, —OPr, —OBu, —OBzand -NMe₂;p is an integer equal to the oxidation state of the metal M minus 2;n is 0 or 1; when n is 0 the bridge L is not present;L is a divalent hydrocarbon moiety containing from 1 to 40 carbon atoms,optionally containing up to 5 silicon atoms, bridging Cp and A,preferably L is a divalent group (ZR⁷ ₂)_(n);Z being C, Si, and the R⁷ groups, equal to or different from each other,being hydrogen or a hydrocarbon radical containing from 1 to 40 carbonatoms;more preferably L is selected from Si(CH₃)₂, SiPh₂, SiPhMe, SiMe(SiMe₃),CH₂, (CH₂)₂, (CH₂)₃ or C(CH₃)₂;Cp is a substituted or unsubstituted cyclopentadienyl group, optionallycondensed to one or more substituted or unsubstituted, saturated,unsaturated or aromatic rings;A has the same meaning of Cp or it is a NR⁷, —O, S, moiety wherein R⁷ isa hydrocarbon radical containing from 1 to 40 carbon atoms;

Alumoxanes used as component b) are considered to be linear, branched orcyclic compounds containing at least one group of the type:

wherein the substituents U, same or different, are defined above.

In particular, alumoxanes of the formula:

can be used in the case of linear compounds, wherein n¹ is 0 or aninteger of from 1 to 40 and where the U substituents, same or different,are hydrogen atoms, halogen atoms, C₁-C₂₀-alkyl, C₃-C₂₀-cyclalkyl,C₆-C₂₀-aryl, C₇-C₂₀-alkylaryl or C₇-C₂₀-arylalkyl radicals, optionallycontaining silicon or germanium atoms, with the proviso that at leastone U is different from halogen, and j ranges from 0 to 1, being also anon-integer number; or alumoxanes of the formula:

can be used in the case of cyclic compounds, wherein n² is an integerfrom 2 to 40 and the U substituents are defined as above.

The catalyst may suitably be employed in the form of a pre-polymerpowder prepared beforehand during a pre-polymerization stage with theaid of a catalyst as described above. The pre-polymerization may becarried out by any suitable process, for example, polymerization in aliquid hydrocarbon diluent or in the gas phase using a batch process, asemi-continuous process or a continuous process.

According to another object, the present invention provides an apparatusfor the polymerization of olefins in the gas phase, which comprises agas-phase polymerization reactor having interconnected polymerizationzones, the reactor being composed of a riser (1) through which thepolymer particles flow upwards under fast fluidization or transportconditions, and a downcomer (2) through which the polymer particles flowdownward in a densified form under the action of gravity; such reactorbeing provided with:

-   -   (i) a restriction (9) positioned in the lower part of the        downcomer in a restriction zone extending from the bottom of the        downcomer upward to a distance of 15% of the portion of the        downcomer occupied by a densified bed of polymer particles, and    -   (ii) a feed line (11) connected to the downcomer at a feed point        being located in a feed zone extending from the top of the        restriction upward to a distance five times the diameter of the        section of the downcomer above the restriction, such feed line        (11) being connected to an antistatic agent feed line (19).

The apparatus can comprise one or more other feed lines (22) formetering the antistatic agent to additional positions along the heightof the downcomer.

The following examples will further illustrate the present inventionwithout limiting its scope

EXAMPLES Methods

The characterization data for the propylene polymers were obtainedaccording to the following methods:

Melt flow rate (MFR)—determined according to ISO 1133 (230° C., 2.16Kg).

Ethylene content—determined by IR spectroscopy

Xylene Solubles (XS)—determined as follows: 2.5 g of polymer and 250 mlof xylene are introduced in a glass flask equipped with a refrigeratorand a magnetical stirrer. The temperature is raised in 30 minutes up tothe boiling point of the solvent. The clear solution so obtained is thenkept under reflux and stirring for further 30 minutes. The closed flaskis then kept in thermostatic water bath at 25° C. for 30 minutes. The soformed solid is filtered on quick filtering paper. 100 ml of thefiltered liquid is poured in a previously weighed aluminium container,which is heated on a heating plate under nitrogen flow, to remove thesolvent by evaporation. The container is then kept on an oven at 80° C.under vacuum until constant weight is obtained. The weight percentage ofpolymer soluble in xylene at room temperature is then calculated.

Example 1

The process of the invention was carried out under continuous conditionsin a plant comprising a gas-phase polymerization reactor havinginterconnected polymerization zones, as shown in FIG. 1.

A Ziegler-Natta catalyst was used as the polymerization catalystcomprising:

-   -   a titanium solid catalyst component prepared with the procedure        described in EP 728 769, Example 5, lines 46 to 53, according to        which di-isobutyl phthalate is used as an internal donor        compound;    -   triethylaluminium (TEAL) as a cocatalyst;    -   dicyclopentyldimethoxysilane as an external donor.

The above catalyst components were pre-contacted in a pre-activationvessel at a temperature of 15° C. for 10 minutes with a weight ratioTEAL/(solid catalyst component) of 4 and a weight ratio TEAL/(externaldonor) of 4.

The thus activated catalyst was fed to the gas-phase polymerizationreactor, where propylene was polymerized with ethylene using H₂ asmolecular weight regulator and propane as an inert polymerizationdiluent. The polymerization was carried out at a temperature of 75° C.and at a pressure of 28 bar. Plant capacity was 19.3 t/h.

An antistatic agent (ATMER 1630) was metered into the reactor at severallocations at a ratio of 130 ppm per kg/h of polymer produced. Withreference to FIG. 1, about 32% of the antistatic agent was metered intothe lower part of the downcomer via line 11 (flow A1), while theremainder 68% was metered via lines 22, 25 and 27 (flows A2, A3 and A4).

This configuration of the antistatic feed ensured a very stableoperation of the plant for the duration of the trial, as evidenced bythe absence of problems at the reactor discharge, and as highlighted bystable and normal values of the temperatures detected on the outsidereactor walls at the bottom of the downcomer, ranging from 91.8 to 96°C.

The obtained polymer had MFR of 10.5 g/10′, ethylene comonomer contentof 3.2% and Xylene Solubles of 5.5%.

Example 2 Comparative

The operation according to example 1 was continued with the only changethat the antistatic flow rate A1 metered through line 11 was stopped andredirected to the other streams A2, A3 and A4 so that 100% of theantistatic was metered via lines 22, 25 and 27. This had a sudden andunwanted effect on the skin temperatures of the downcomer which in a fewminutes increased abruptly to 107.1° C. Most importantly, this wasfollowed in a short time by the pluggage of the reactor discharges andthe eventual shutdown.

1. A process for producing olefin polymers, which comprises the stepsof: a. polymerizing one or more olefins in the gas phase, in thepresence of an olefin polymerization catalyst, whereby growing polymerparticles flow along a cylindrically-shaped downward path in densifiedform under the action of gravity so as to form a densified bed ofdownward-flowing polymer particles; b. allowing said polymer particlesto flow through a restriction of the densified bed, such restrictionbeing positioned in a restriction zone extending from the bed bottomupward to a distance of 15% of the total height of the densified bed;and c. metering an antistatic agent through a feed line connected to thedensified bed at a feed point being located in a feed zone extendingfrom the top of the restriction upward, to a distance five times thediameter of the section of the densified bed above the restriction. 2.The process according to claim 1, wherein the restriction of the bed ispositioned in a zone extending from the bed bottom upward to a distanceof 10%, preferably 5%, of the total height of the densified bed.
 3. Theprocess according to claim 1, wherein the feed point line for meteringthe antistatic agent is located above the restriction, in a feed zoneextending from said upward to a distance to of preferably 4 times, morepreferably 3 times, even more preferably 2 times the diameter of thesection of the densified bed immediately above the restriction.
 4. Theprocess according to claim 1, wherein a stream of a dosing gas is fedinto the lower part of the densified bed by means of a feed line placedat a distance above the restriction of up to 1.5 times, preferablycomprised between 0.6 and 1.3 times, more preferably between 0.7 and 1.0times. the diameter of the section of the densified bed immediatelyabove the restriction.
 5. The process according to claim 4, wherein thefeed line for metering the antistatic agent and the feed line formetering the dosing gas are coincident.
 6. An apparatus for thepolymerization of olefins in the gas phase, which comprises a gas-phasepolymerization reactor having interconnected polymerization zones, thereactor comprising a riser (1) through which the polymer particles flowupwards under fast fluidization or transport conditions and a downcomer(2) through which the polymer particles flow downward in a densifiedform under the action of gravity, the reactor being provided with: a. arestriction (9) positioned in the lower part of the downcomer in arestriction zone extending from the bottom of the downcomer upward to adistance of 15% of the portion of the downcomer occupied by a densifiedbed of polymer particles, and b. with a feed line (11) connected to thedowncomer at a feed point being located in a feed zone extending fromthe top of the restriction upward to a distance five times the diameterof the section of the downcomer above the restriction, such feed line(11) being connected to an antistatic agent feed line (19).
 7. Theapparatus according to claim 6, comprising one or more other feed lines(22) for metering the antistatic agent to additional positions along theheight of the downcomer.